Method for the production of polybutylene terephthalate

ABSTRACT

In a process of producing PBT, a mixture of BDO and TPA is esterified in the presence of a catalyst solution, and the esterification product is polycondensated. To avoid the formation of deposits in the PBT, the catalyst solution contains 0.05 to 10 wt-% alkyl titanate, 85 to 99 wt-% BDO, 50 to 50,000 ppm bifunctional carboxylic acid equivalents and/or monofunctional hydroxycarboxylic acid equivalents and not more than 0.5 wt-% water.

DESCRIPTION

[0001] This invention relates to a process of producing polybutyleneterephthalate (PBT) from butanediol (BDO) and terephthalic acid (TPA) orfrom butanediol (BDO) and dimethyl terephthalate (DMT), in which amixture of BDO and TPA or BDO and DMT is subjected to an esterificationor transesterification and subsequently to a polycondensation, in thepresence of a catalyst solution containing alkyl titanate attemperatures in the range from 130 to 260° C.

[0002] It is known that PBT is produced from TPA or DMT by reaction withBDO by using catalysts. From among the multitude of catalysts used, inparticular alkyl titanates, preferably tetrabutyl orthotitanate andtetraisopropyl orthotitanate, have gained acceptance due to theirefficiency and large-scale availability (Ullmann's Encyclopedia ofIndustrial Chemistry, 5th edition, vol. A 21, page 237). The alkyltitanates have properties which have a disadvantageous effect in theproduction of PBT, because they tend towards hydrolysis and inparticular in the process of producing PBT on the basis of TPA forminsoluble particles during esterification, which particles not onlyreduce the service life of the filters for the PBT melt, but also impairthe quality of the PBT produced. As a result of their volatility,thermal decomposability and easy reducibility, alkyl titanates can alsoform titanium-containing dark deposits at the reactor walls and in theprocess column, which impair the quality of the PBT produced.

[0003] To avoid the negative effects of alkyl titanate during theproduction of PBT, various stabilization methods for the catalysts areknown (WO-A-99/45056), in that additional substances foreign to theprocess are added to said catalysts, which substances in turn involvecertain disadvantages.

[0004] It is the object underlying the invention to provide a catalystsolution of improved reactivity, which is resistant to hydrolysis andstable in storage and does not form any deposits during the productionof PBT, in particular also during esterification. Moreover, negativeinfluences on the PBT produced as well as disturbing residues on theproduct melt filters should be avoided.

[0005] This object is solved in that the catalyst solution contains 0.05to 10 wt-% alkyl titanate, 85 to 99 wt-% BDO, 50 to 50,000 ppm,preferably 50 to 30,000 ppm dicarboxylic acid and/or hydroxycarboxylicacid and/or monohydroxyalkyl dicarboxylic acid and not more than 0.5wt-% water. Such catalyst solution forms a clear, particle-free mixtureof the above-mentioned components.

[0006] The catalyst solution can be used both for the continuous and forthe discontinuous production of PBT by esterification of TPA with BDOand also for the transesterification of DMT with BDO. Under inertconditions, the catalyst solution can permanently be stored attemperatures above 25° C. in the liquid condition and at temperaturesbelow 20° C. also in the solid condition. A decisive advantage of thecatalyst solution is the fact that its excellent stability is based onthe use of process-conformal components which are completelyincorporated in the PBT produced and are not left as disadvantageousbyproduct. What turned out to be particularly advantageous is the use ofTPA or isophthalic acid in BDO as solubilizer for alkyl titanate, as thepolymer properties of PBT are not or only insignificantly influencedthereby.

[0007] The production of the catalyst solution is effected in that firstof all a preliminary solution is produced by mixing BDO with alkyltitanate at temperatures of 50 to 230° C., and dicarboxylic acid and/orhydroxycarboxylic acid and/or hydroxyalkyl dicarboxylic acid monoesterare added to this preliminary solution.

[0008] Another possibility for producing the catalyst solution consistsin that first of all a preliminary solution is produced by mixing BDOwith dicarboxylic acid and/or hydroxycarboxylic acid and/ormonohydroxyalkyl dicarboxylic acid at temperatures of 50 to 230° C. andalkyl titanate is added to this preliminary solution. The catalystsolutions should only have a limited water content of not more than 0.5wt-%, as with a larger water content turbidities or precipitations mayoccur during the further processing. The catalyst solutions have aturbidity value NTU (Normal Turbidity Unit) of ≦1. The NTU value, whichis a measure for the turbidity of solutions, is determined for a 10 wt-%solution of polyester in phenol/dichlorobenzene (3:2 parts by weight) inthe nephelometer of the firm Hach (type XR, according to U.S. Pat. No.4,198,161) with a cuvette having a diameter of 22.2 mm, analogously tothe DIN standard 38 404, part 2, commonly used for water. There ismeasured the intensity of the scattered light as compared to a standardformalin solution minus the value of the solvent (about 0.3 NTU).Solutions with NTU values ≦2 are completely clear; with NTU values >2turbidities occur more and more.

[0009] For producing the catalyst solutions, both aromatic and aliphaticdicarboxylic acids can be used singly or several of them as a mixture.Aromatic dicarboxylic acids are for instance TPA and isophthalic acid.The aliphatic dicarboxylic acids include for instance oxalic acid,malonic acid, succinic acid and adipic acid. The dicarboxylic acids canwholly or partly be replaced by aromatic and aliphatic hydroxycarboxylicacids and/or monohydroxyalkyl dicarboxylic acid, in particularmonohydroxybutyl terephthalic acid. Aromatic hydroxycarboxylic acidsare, for instance, 3-hydroxybenzoic acid and 4-hydroxybenzoic acid.Aliphatic hydroxycarboxylic acids include, for instance, hydroxybutyricacid, hydroxypropanoic acid and glycolic acid. Mixtures of thesehydroxycarboxylic acids may also be used. Monohydroxybutyl terephthalicacid is formed in the process from TPA and BDO. Tetrabutyl orthotitanateand tetraisopropyl titanate turned out to be particularly useful asalkyl titanate.

[0010] For producing PBT it is possible to meter the catalyst solutionin the process at one point or also simultaneously at several points.The catalyst solution can, for instance, already be added to the pasteformulation or to a subsequent process stage or to a melt transfer line.It is also possible to feed the catalyst solution into the reflux of BDOfrom the process column to the first process stage and possibly into asubsequent process stage.

[0011] A particularly advantageous embodiment of the process inaccordance with the invention is the direct production of the catalystsolution during the process of producing PBT, wherein alkyl titanate istransferred into the catalyst solution in a mixing line with theterephthalic acid reflux of BDO from the process column to the firstesterification stage and/or further esterification stages. In this case,the reflux of BDO includes the preliminary solution in accordance withthe invention.

[0012] In the drawing, the process of producing PBT from BDO and TPA isrepresented by way of example as a a flow diagram of the apparatus andwill subsequently be explained in detail:

[0013] To a stirred tank (1), TPA is supplied via line (2) and BDO issupplied via line (3) and mixed to form a paste which via line (4) isfed into a stirred reactor (7) surrounded by a heating jacket (5) andequipped with an additional heating element (6), for esterification.During esterification, a liquid BDO/oligomer mixture and vapor areformed, the latter substantially consisting of water, BDO, andtetrahydrofuran (THF). In the case of the transesterification of DMT inthe stirred reactor (7), the vapor formed substantially containsmethanol, BDO, THF and water. The esterification in the stirred reactor(7) is effected in the presence of the catalyst solution supplied vialine (8). The vapor formed in the stirred reactor (7) leaves the stirredreactor (7) via line (9) and is supplied to the distillation column(10), in which water and THF are separated overhead. The overheadproduct of the distillation column (10) is supplied via line (11) to thecooler (12) from which the condensate is discharged via line (13) to thereflux distributor (14). From the reflux distributor (14), water and THFare withdrawn via line (15), and the reflux is recirculated to the topof the distillation column (10) via line (16). From the bottom of thedistillation column, a mixture consisting of higher-boiling components,usually containing 90 to 99 wt-% BDO and 50 to 3000 ppm dicarboxylicacid and monohydroxyalkyl dicarboxylic acid, which were obtained bymonomer evaporation and possibly by subsequent hydrolysis of esterbonds, is withdrawn via line (17). The larger part of the mixture isdirectly supplied to the stirred reactor (7) via line (18), whereas tothe smaller part of the mixture metered via line (17) alkyl titanate isadded via line (18), so that the composition desired for the catalystsolution is obtained. In the case of the transesterification of DMT,alkyl titanate and in addition dicarboxylic acid or hydroxycarboxylicacid in a weight ratio titanium:acid of 10:1 to 1:10 shouldadvantageously be fed into line (17) as preliminary solution suppliedvia line (18). The mixture of higher-boiling components and catalystsolution passes through a mixing line (19), in which homogenization isachieved by static mixing elements. Via line (8), the finished catalystsolution reaches the reflux of BDO flowing in line (18) and is thensupplied to the stirred reactor (7).

[0014] It is also possible to wholly or partly add catalyst solutions tothe stirred tank (1) at the metering point (20), and/or feed the same atthe metering point (21) into the paste line (4) and/or at the meteringpoint (22) into the transfer line (13) between the stirred reactor (7)and the likewise stirred precondensation reactor (24). The reactor (24)has a heating jacket (25) and an additional heating element (26). Theprepolymer from the stirred reactor (24) is supplied to thepolycondensation reactor (28) via line (27). The PBT produced in saidpolycondensation reactor is pumped off via line (29). Via line (30), thevapors formed are sucked off from the stirred reactor (24) and via line(31) from the reactor (28). The temperatures in the reactors (7, 24, 28)lie in the range from 130 to 260° C.

[0015] The process in accordance with the invention will subsequently beexplained in detail by means of several embodiments.

[0016] 1.0 Preparation of Preliminary Solutions for the CatalystSolutions

[0017] 1.1 97.85 wt-% BDO with a water content of 0.3 wt-% are mixedwith 0.05 wt-% TPA and heated to a temperature of 80° C. by stirring,are maintained at this temperature for 30 min and are then cooled toroom temperature. The preliminary solution is clear, no precipitates areformed, and the NTU value is 0.36. When storing the preliminary solutionhermetically sealed at a temperature of 25° C., said solution maintainsits consistency for a period of 14 days, as is shown by an unchanged NTUvalue.

[0018] For the case that the preliminary solution is produced withoutheating to a temperature of 80° C., precipitates are formed within 24hours.

[0019] 1.2 96.5 wt-% BDO with a water content of 0.15 wt-% are mixedwith 1.5 wt-% isophthalic acid and heated to a temperature of 80° C. bystirring, are maintained at this temperature for 30 min and are thencooled to room temperature. The preliminary solution is clear; noprecipitates are formed. The NTU value is 0.35. When storing thepreliminary solution hermetically sealed at a temperature of 25° C., theconsistency of the preliminary solution is maintained for a period of 14days. The NTU value remains unchanged.

[0020] 2.0 Preparation of Catalyst Solutions

[0021] 2.1 To the preliminary solution produced in accordance withExample 1, 2.1 wt-% tetrabutyl orthotitanate are added by stirring at atemperature of 40° C., and stirring is continued for one hour at atemperature of 80° C. The catalyst solution formed is clear, has ayellow inherent color and an NTU value of 0.50. The catalyst solutionstored hermetically sealed at a temperature of 25° C. for a period of 14days maintains its consistency, which is demonstrated by an NTU value of0.44.

[0022] 2.2 At a temperature of 40° C., 2.1 wt-% tetrabutyl orthotitanateare added by stirring to the preliminary solution produced according toExample 1.2, and stirring is continued for one hour at a temperature of80° C. The catalyst solution thus produced maintains its consistency ata storage temperature of 25° C. and hermetically sealed for a period of14 days, as is shown by the NTU value of 0.91.

[0023] 2.3 At a temperature of 40° C., 7.1 wt-% tetrabutyl orthotitanateare added by stirring to the preliminary solution produced according toExample 1.2, and stirring is continued for one hour at a temperature of80° C. The catalyst solution produced is clear and maintains itsconsistency at a storage temperature of 25° C. for a period of 14 days.The NTU value is 0.89.

[0024] 2.4 97.4 wt-% BDO are mixed with 2.1 wt-% tetrabutylorthotitanate and heated to 80° C. by stirring. Then, 0.5 wt-% TPA areadded, heated to a temperature of 160° C. by stirring and maintained atthis temperature for one hour, TPA being dissolved completely. Uponcooling to room temperature, the catalyst solution has an NTU value of0.66. When stored hermetically sealed for a period of 14 days at atemperature of 25° C., the catalyst solution maintains its consistency.The NTU value remains unchanged.

[0025] The same clear catalyst solutions are obtained when instead ofTPA the same amount of p-hydroxybenzoic acid or malonic acid is used;samples of this catalyst solution have an NTU value of 0.69 forp-hydroxybenzoic acid and an NTU value of 0.79 for malonic acid.

[0026] 2.5 To a sample of the reflux of BDO from the bottom of theprocess column of a continuous plant for esterifying TPA and BDO, whichdue to the measured acid number contains 3000 ppm TPA equivalents, 2wt-% tetrabutyl orthotitanate are added at a temperature of 85° C. Thecatalyst solution formed is clear with an NTU value of 0.51; itmaintains its consistency when stored for a period of 14 days at atemperature of 25° C.

[0027] 2.6 Into the reflux of BDO from the bottom of the process column,which has a temperature of 190° C. and contains 3000 ppm TPAequivalents, tetrabutyl orthotitanate with a temperature of 25° C. ismetered in an amount of 0.07 wt-% and mixed with the BDO in a mixingline comprising 6 static mixers. The resulting catalyst solution is fedinto the esterification reactor as modified reflux from the processcolumn, and there is adjusted a titanium concentration of 100 ppm basedon PBT. This catalyst solution has an NTU value of 0.57, its consistencyis maintained over a storage period of 14 days at a temperature of 25°C.

[0028] The stability of the catalyst solutions prepared in accordancewith Examples 2.1, 2.2, 2.3 and 2.4 under the influence of moisture istested at room temperature in that samples of catalyst solutions of 100g each and with a temperature of 25° C. are mixed with 0.5 g, 1.2 g and5.0 g water, so that the water content of BDO is increased from aninitial water content of 0.3 wt-% to 0.8 wt-%, 1.5 wt-% and 6.3 wt-%.When stored hermetically sealed for a period of 14 days at a temperatureof 25° C., the catalyst solutions with a water content of 0.8 wt-%remain unchanged. It was furthermore revealed that catalyst solutionswhich have a water content >1 wt-% become turbid after 2 hours already,and an irreversible precipitate is formed within 4 days.

[0029] 3.0 Discontinuous Production of PBT Prepolymer

[0030] 3.1 A stirred reactor having a capacity of 5 1 is charged with22.5 g of a catalyst solution prepared according to Example 2.2, 1127 gTPA (including the amount dissolved in the catalyst solution), and 1813g BDO (including the BDO contained in the catalyst solution). Uponinertization with nitrogen for three times, the charge is heated to atemperature of 235° C. within 2 hours by stirring at normal pressure andis esterified at this temperature for 2 hours at a pressure of 400 mbar.For a total period of 100 min, a precondensation is then effected at atemperature of 240° C., a pressure of 50 mbar and a stirring rate of 100min⁻¹.

[0031] Upon termination of the precondensation, the melt is pressed outfrom the reactor as a jet by means of nitrogen, collected on a pancooled with liquid nitrogen, and solidified. For analysis, the productis ground and its intrinsic viscosity and filter load value aredetermined. The values determined are listed in Table 1 below.

[0032] The intrinsic viscosity (IV) is measured at 25° C. in a solutionof 500 mg polyester in 100 ml of a mixture of phenol and 1,2dichlorobenzene (3:2 parts by weight) and constitutes a measure for themolecular weight of the sample.

[0033] The filtration behavior of the product melts is obtained bymeasuring the filter load value characteristic of the purity of theproducts, in that the PBT prepolymer dried for 13 hours at a temperatureof 130° C. and under a reduced pressure of <1 mbar is molten in alaboratory extruder and pressed through a disk filter with a mesh sizeof 15 μm and a filter area of 2.83 cm² by means of a gear pump at atemperature of 260° C. The increase in pressure before the filter isrecorded with respect to the amount of melt conveyed, and thefilterability is calculated as filter load value (FLV): FLV=increase inpressure Δp [bar]·filter area [cm²]/amount of melt [kg].

[0034] 3.2 For the discontinuous production of PBT prepolymer, acatalyst solution produced according to Example 2.3 is used underotherwise the same process conditions as in Example 3.1. The values forthe intrinsic viscosity and for the filter load are indicated in Table1.

[0035] 3.3 For the discontinuous production of PBT prepolymer, TPA andBDO are used together with 76.6 g catalyst solution when usingp-hydroxybenzoic acid in accordance with Example 2.4, as described inExample 3.1. The product produced has the intrinsic viscosity and filterload values as indicated in Table 1.

[0036] 3.4 A stirred reactor having a capacity of 5 1 is charged with13.2 g of a catalyst solution prepared according to Example 2.3, 1316 gDMT and 900 g BDO (including the BDO in the catalyst solution). Uponinertization with nitrogen for three times, the charge is heated to atemperature of 150° C. within one hour by stirring at normal pressure,maintained at this temperature for 30 min, and subsequently heated to atemperature of 235° C. within 2 hours and transesterified by removingthe methanol. Thereupon, the transesterification product is precondensedfor a period of 100 min at a temperature of 240° C., a pressure of 50mbar and a stirring rate of 100 min⁻¹. Upon termination of theprepolycondensation, the melt is pressed out from the reactor as a jetby means of nitrogen and collected on a pan cooled with liquid nitrogen.The intrinsic viscosities and the filter load values of the prepolymersare indicated in Table 1.

[0037] 3.5 For comparison purposes, tetrabutyl orthotitanate as catalystsolution is directly mixed with BDO at room temperature for thediscontinuous production of PBT prepolymer under otherwise the sameprocess conditions as in Example 3.1. The value determined for theintrinsic viscosity of the prepolymer is unchanged, whereas the filterload is increased by a multiple, as is represented in Table 1. TABLE 1Prepolymer analyses Example 3.1 3.2 3.3 3.4 3.5 IV dl/g 0.168 0.1700.174 0.174 0.165 Filter load value bar · cm²/kg 5 7 6 6 138

[0038] 4.0 Discontinuous Polycondensation of PBT Prepolymers

[0039] Subsequent to the production of PBT prepolymers in the stirredreactor having a capacity of 5 1, a polycondensation is performed at atemperature of 245° C. and a vacuum of 0.5 mbar for a total period of180 min. The initial rotational speed is 100 min⁻¹; upon reaching acertain reference momentum, the rotational speed is reduced to about 30min⁻¹ with this constant momentum, so that comparable values for theintrinsic viscosities are achieved. Subsequent to the polycondensation,the product melt is pressed out from the stirred reactor as a strand,passed through a cooling vat and cut into chips. Table 2 below indicatesthe measurement results for the intrinsic viscosities and the filterload of these polymers produced from the prepolymers in accordance withExamples 3.1, 3.2, 3.3, 3.4 and 3.5. There are also indicated theresults of the determination of terminal COOH groups.

[0040] The concentration of terminal COOH groups is determined by meansof photometric titration with 0.05% ethanolic potassium hydroxidesolution against bromothymol blue of a solution of a polyester in amixture of o-cresol and chloroform (70:30 parts by weight). TABLE 2Polymer analyses Prepolymers from Example 3.1 3.2 3.3 3.4 3.5 IV dl/g0.847 0.860 0.855 0.852 0.860 Filter load bar · cm²/kg 10 9 11 10 98value Terminal meq/kg 12 13 13 12 13 COOH groups

[0041] 5.0 Continuous Production of PBT Prepolvmer with SubsequentDiscontinuous Polycondensation

[0042] 5.1 In a paste mixer, TPA and BDO are pasted in a molar ratio of1:1.5 and via a gear pump stationarily metered into a jacket-heatedfirst stirred reactor in an amount of 2.66 kg/h. In addition, catalystsolution produced in accordance with Example 2.5 with a content of 120ppm titanium based on the polymer together with 1.193 kg/h mol BDO basedon TPA is fed into the bottom of the first reactor. The dwell time ofthe esterification product in the first reactor is 80 min, thetemperature is 245° C. and the pressure is 400 mbar by maintaining aconstant filling level in the first reactor. The resulting vapor ofwater, BDO and THF is continuously withdrawn overhead, condensed incooled pre-evacuated recipients and removed periodically. Via aconnecting line with throttle valve, the esterification product issupplied from the first reactor to a second stirred reactor, in which itis prepolycondensated during a dwell time of 30 min at a temperature of250° C. and a pressure of 50 mbar by maintaining a constant fillinglevel. The vapor consisting of BDO and lower-boiling components, whichwas formed in the second reactor, is continuously withdrawn overhead,collected in cooled recipients and discharged periodically. Theprepolymer formed in the second reactor is discharged via a gear pump inan amount of 2 kg/h, collected in metal tanks cooled with liquidnitrogen, which are changed every hour, and is frozen. The prepolymercharacteristics are 0.256 dl/g for the intrinsic viscosity and 18bar·cm²/kg for the filter load.

[0043] 5.2 For comparison purposes, 2.1 wt-% tetrabutyl orthotitanateare used at room temperature instead of catalyst solution in theabove-described continuous production of PBT prepolymer. The analyses ofthe prepolymer revealed an intrinsic viscosity of 0.257 dl/g and afilter load of 293 bar·cm²/kg.

[0044] 5.3 Upon freezing, grinding and melting, the PBT prepolymerproduced continuously in accordance with Example 5.1 is subjected to adiscontinuous polycondensation within 1 hour, as described in Example4.0. On a sample of the polymer produced in this way, an intrinsicviscosity of 0.856 dl/g, a filter load of 15 bar·cm²/kg and an amount ofterminal COOH groups of 14 meq/kg is determined.

[0045] 5.4 The PBT prepolymer produced in accordance with Example 5.2 issubjected to the same experimental conditions as they are represented inExample 4.0. The polymer characteristics are 0.862 dl/g for theintrinsic viscosity, 308 bar·cm²/kg for the filter load and 15 meq/kgfor the terminal COOH groups.

[0046] The filter load values determined for the prepolymers and thepolymers reveal that with the catalyst solutions composed in accordancewith the invention a PBT esterification or DMT transesterification isensured without the occurrence of precipitates.

1. A process of producing polybutylene terephthalate (PBT) frombutanediol (BDO) and terephthalic acid (TPA), in which a mixture of BDOand TPA is subjected to an esterification in the presence of a catalystsolution containing alkyl titanate at temperatures in the range from 130to 260° C. and the product of the esterification is subjected to apolycondensation, characterized in that the catalyst solution contains0.05 to 10 wt-% alkyl titanate, 85 to 99 wt-% BDO, 50 to 50,000 ppmdicarboxylic acid and/or hydroxycarboxylic acid and/or monohydroxyalkyldicarboxylic acid and not more than 0.5 wt-% water.
 2. A process ofproducing polybutylene terephthalate (PBT) from butanediol (BDO) anddimethyl terephthalate (DMT), in which a mixture of BDO and DMT issubjected to a transesterification in the presence of a catalystsolution containing alkyl titanate and the product of thetransesterification is subjected to a polycondensation, characterized inthat the catalyst solution contains 0.05 to 10 wt-% alkyl titanate, 85to 99 wt-% BDO, 50 to 50,000 ppm dicarboxylic acid and/orhydroxycarboxylic acid and/or monohydroxyalkyl dicarboxylic acid and notmore than 0.5 wt-% water.
 3. The process as claimed in any of claims 1and 2, characterized in that the content of dicarboxylic acid and/orhydroxycarboxylic acid and/or monohydroxyalkyl dicarboxylic acid is 50to 30,000 ppm.
 4. The process as claimed in any of claims 1 to 3,characterized in that BDO is mixed with alkyl titanate at temperaturesof 50 to 230° C. and dicarboxylic acid and/or hydroxycarboxylic acidand/or monohydroxyalkyl dicarboxylic acid is added to this mixture. 5.The process as claimed in any of claims 1 to 3, characterized in thatBDO is mixed with dicarboxylic acid and/or hydroxycarboxylic acid and/ormonohydroxyalkyl dicarboxylic acid at temperatures of 50 to 230° C. andalkyl titanate is added to this mixture.
 6. The process as claimed inclaim 1, characterized in that a vapor mixture containing BDO, water anddicarboxylic acid is withdrawn from the esterification stage, waterpossibly together with low-boiling components is separated bydistillation, and alkyl titanate is admixed to the higher-boilingcondensate at a temperature of 50 to 230° C.
 7. The process as claimedin any of claims 1 to 6, characterized in that as dicarboxylic acidthere is used one or more in a mixture of terephthalic acid, isophthalicacid, oxalic acid, malonic acid and succinic acid.
 8. The process asclaimed in any of claims 1 to 6, characterized in that ashydroxycarboxylic acid there is used one or more in a mixture of3-hydroxybenzoic acid, 4-hydroxybenzoic acid, 3-hydroxybutyric acid,hydroxypropanoic acid and glycolic acid.
 9. The process as claimed inany of claims 1 to 6, characterized in that monohydroxybutylterephthalic acid is used as monohydroxyalkyl dicarboxylic acid.
 10. Theprocess as claimed in any of claims 1 to 9, characterized in thattetrabutyl orthotitanate is used as alkyl titanate.
 11. The process asclaimed in any of claims 1 to 10, characterized in that catalystsolution is additionally introduced in at least one polycondensationstage subsequent to the esterification stage.